Now that there are two airlines and a number of routes, flying to Santo is a little more confusing than it used to be. Here is some information to make it easier for you:
1. Fly domestic to Brisbane, then Pacific Blue to Vila, then domestic to Santo.
Pacific Blue looks the cheapest on paper, but it works out more expensive now overall because you don't get a discount on the domestic flights in Vanuatu. They are also very strict on excess baggage and don't provide food etc. We stay away from it now because it is an extra flight, more expensive and poorer service. If you live in Brisbane, it might be OK, but the option #2 flight direct to Santo would be preferable.
2. Fly domestic from Sydney to Brisbane, then Air Vanuatu direct to Santo.
TO VANUATU (Lugavnille, Pekoa Airport): SUNDAYS (10am-1:40pm, NF21)
BACK TO AUSTRALIA: MONDAYS (5:45pm-7:45pm, NF21)
This is good for coming to Vanuatu in NSW daylight saving time, but once daylight saving has finished the plane leaves Brisbane too early on Sunday morning so you can't fly out of Sydney on Sunday. You would need to stay in Brisbane Saturday night, so it becomes more expensive.
It may still work when you return to Australia, if you want to go back on a Monday. I'm not sure. This option is slightly cheaper than number 3.
3. Fly from Sydney to Santo (with transit in Vila) on Air Vanuatu.
TO VANUATU (Luganville, Pekoa Airport): MONDAYS 11:50am
BACK TO AUSTRALIA (Sydney): SUNDAYS 2:10pm?
This is by far the easiest to do. You can check in your bags at Sydney and they go all the way through to Santo. The overall time is much less because it is only a very short transit in Vila. Also, you don't have to pay excess baggage again in Vila for the domestic flight because you bags are booked all the way through. They take about $100 off the ticket price because they don't have to do baggage handling in Vila (compared to #4). Overall this seems to be the best option by far, and it is only slightly more expensive than number 2 or similar in price. But it only goes one day each week.
4. Fly from Sydney to Vila on Air Vanuatu, then get a domestic flight to Santo.
This used to be the only option. It is more expensive than the new options (#2 and #3). But the service is more regular so we use it when we need to fly on a different day.
TO VANUATU (Port Vila, Bauerford Airport): Best is Friday 11:50am-3:30pm because you can fly straight through to Santo on a small plane without staying in Vila. Other flights are: Wednesday 8:20pm-11:55pm; Thursday 4:50pm-8:30pm; Saturday 10:20pm-11:55pm; Sunday 8:50pm-12:30pm
BACK TO AUSTRALIA (Sydney): Best is Wednesday 3:30pm-7:20pm, Thursday 12:00pm-3:50pm and Saturday 3:30pm-7:20pm because you can fly out of Santo in the morning and not stay in Vila the night.
Other flights are: Monday 7:00am-10:50am and Friday 7:00am-10:50am